Perfusion of heart is a unique in vitro model for studies on rejection of an organ by antibodies. Xenogeneic sera will be used for perfusion, and neutralization of naturally occurring antibodies by means of solubilized cell-membrane antigens will be attempted. These studies should provide important information for clinical application of xenografts. Studies on species-specific cell-surface antigens will be continued using man-mouse hybrid cells with one or a few clearly identified residual human chromosomes. The appearance of circulating immune complexes composed of transplantation antigens and antibodies will be studied in man and animals in order to learn about the role of such complexes in late rejection of renal grafts. Release of tissue antigens and heterophile antigens to the circulation and appearance of their corresponding antibodies will be investigated with the aim of establishing serological diagnosis of graft rejection. Animal experiments will be performed on the effect of transplantation antibodies to minor histocompatibility antigens on pregnancy. Damage to placenta and fetuses will be ascertained.